Pulse - Phase Spectroscopy of Smc X - 1 with Chandra and Xmm - Newton : Reprocessing by a Precessing Disk ?

We present pulse-phase X-ray spectroscopy of the high-mass X-ray pulsar SMC X-1 from five different epochs, using Chandra X-ray Observatory ACIS-S and XMM-Newton EPIC-pn data. The X-ray spectrum consistently shows two distinct components, a hard power law and a soft blackbody with kT BB ∼ 0.18 keV. For all five epochs the hard component shows a simple double-peaked pulse shape, and also a variation in the power law slope, which becomes harder at maximum flux and softer at minimum flux. For the soft component, the pulse profile changes between epochs, in both shape and phase relative to the power law pulses. The soft component is likely produced by reprocessing of the hard X-ray pulsar beam by the inner accretion disk. We have created a model of a twisted inner disk, illuminated by the rotating X-ray pulsar beam, which simulates pulsations in the soft component due to this reprocessing. We find that for some disk and beam geometries, precession of an illuminated accretion disk can roughly reproduce the observed long-term changes in the soft pulse profiles. 1. INTRODUCTION The general picture of accretion onto X-ray pulsars, consisting of a flow in a disk to the magnetosphere and along the dipole field to the magnetic poles of the neutron star, has been known for decades (see Nagase 1989, and references therein). However, despite extensive study, some important aspects of this process remain mysterious. In particular, the details of accretion near the magnetosphere, where the neutron star's magnetic field begins to dominate the flow, is a complex and challenging problem. 1.1. Superorbital variation in X-ray pulsars One way to explore such a flow observationally is with systems where the shadow of the accretion disk causes the X-ray source to vary with time. There are now at least 20 known accreting binary systems that exhibit superorbital periods, some of which are caused by shadowing of the source flux by an accretion disk (see Clarkson et al. 2003, and references therein). Of these systems, a small group of X-ray pulsars (SMC X-1, LMC X-4 and Her X-1) show luminous, pulsing X-ray emission which allows for study of the accretion flow in unusual detail. These systems each show a 30–60 d su-perorbital variation, attributed to a warped, precessing accretion disk (Tananbaum et al. In the low-mass system Her X-1, the precessing disk has been studied extensively. Regular 35-day variations have been observed in the X-ray flux, …